The invention relates to polycrystalline silicon chunks and to methods for producing them.
Polycrystalline silicon serves as starting material for the production of electronic components and solar cells.
It is obtained by thermal decomposition of a silicon-containing gas or of a silicon-containing gas mixture. This process is identified as deposition from the vapor phase (CVD, chemical vapor deposition). On the large scale, this process is realized in reactors called Siemens reactors. The polysilicon here is produced in the form of rods.
The polysilicon rods are uninstalled from the reactor and comminuted by means of manual methods or mechanically. The coarse-chunk polysilicon this produces is comminuted further, using crushers, to give polycrystalline silicon fragments. A suitable crusher, namely a roll crusher, is known from U.S. Pat. No. 7,950,600 B2. The claimed roll crusher comprises a roll which rotates with a shaft and comminutes polycrystalline silicon, said roll consisting of a steel carrier roll and a plurality of hard-metal segments, the hard-metal segments consisting of a cobalt matrix in which tungsten carbide is incorporated, and the hard-metal segments being fastened reversibly with a form fit on the carrier roll.
Following comminution, the chunk polysilicon is typically classified or sorted by a mechanical screening method or by means of optical sorting.
US 2007/0235574 A1 discloses a device for comminuting and sorting polycrystalline silicon, comprising a facility for feeding coarse-chunk polysilicon into a crusher unit, the crusher unit, and a sorting unit for classifying the chunk polysilicon, characterized in that the device is provided with a controller which allows variable adjustment of at least one crushing parameter in the crusher unit and/or of at least one sorting parameter in the sorting unit.
The crusher unit comprises roll crushers or jaw crushers, preferably spiked roll crushers.
By means of the method it is possible specifically to produce cubic chunk polysilicon with a size of 45 to 250 mm that is no longer flowable, and to do so reproducibly and with a high yield.
By using profiled rolls such as spike rolls, a cubic polysilicon chunk is formed that has a size of 45 to 250 mm, with the ratio of the weight of a chunk and the maximum length of a chunk being ≧3 g/mm on average over all the chunks.
US 2010/0001106 A1 discloses a method for producing high-purity classified chunk polysilicon by comminuting and classifying polysilicon from the Siemens process, using a device comprising comminuting tools and a screening device, and cleaning the resultant chunk polysilicon by means of a cleaning bath, characterized in that the comminuting tools and the screening device consistently possess a surface that comes into contact with the polysilicon and that comprises a material which contaminates the chunk polysilicon only with extraneous particles which can be subsequently removed selectively by the cleaning bath.
Comminution takes place in a crushing unit which comprises a roll crusher or a jaw crusher, preferably a spiked-roll crusher.
Advantages are promised by this method especially when comminuting tools and screening device have a surface made of low-alloy steel.
By use of spiked-roll crushers, cubic polycrystalline silicon chunks are obtained. Metallic impurities can be lowered to a low level through use of comminuting tools made from low-contamination materials such as tungsten carbide.
Nevertheless, low-contamination materials cannot be used for all parts of the units—crushing shells with spikes are stated explicitly. In view of its high brittleness and low toughness, low-abrasion tungsten carbide is not very suitable for use for profiled rolls.
If low-abrasion tungsten carbide is used, as ought to be possible only in the case of smooth rolls, the resulting chunk is non-cubic and has increased tungsten levels, which is a disadvantage.
U.S. Pat. No. 7,270,706 B2, however, discloses a profiled roll having teeth disposed around the circumference of the roll, a shaft on which the roll is rotatably mounted, a housing having surfaces defining a cavity inside which the roll is disposed and/or fastened, an inlet port on top of the housing, an exit port on the bottom of the housing, a plate inside the housing opposite the roll, where the roll, teeth, plate, and housing surfaces defining the cavity are produced from or coated with a material that minimizes contamination of polycrystalline silicon. The material is preferably selected from the group consisting of carbides, cermets, ceramics, and combinations thereof. Particularly preferred is the use of a material selected from the group consisting of tungsten carbide, tungsten carbide with a cobalt binder, tungsten carbide with a nickel binder, titanium carbide, Cr3C2, Cr3C2 with nickel-chromium alloy binder, tantalum carbide, niobium carbide, silicon nitride, silicon carbide in a matrix, such as Fe, Ni, Al, Ti, or Mg, for example, aluminum nitride, tantalum carbide, niobium carbide, titanium carbide with cobalt carbonitride and titanium carbonitride, nickel, nickel-cobalt alloy, iron, and combinations thereof.
It has been found that for a profiled roll made of tungsten carbide or tungsten carbide with a cobalt binder or tungsten carbide with a nickel binder it is impossible to attain high durability. After comminution of just 100 metric tons of silicon, teeth break off from the roll. Accordingly, in order to be able to continue producing cubic polysilicon chunk, the rolls must be replaced, which is expensive and therefore disadvantageous. It would be desirable to be able to comminute at least 1000 metric tons of silicon before the roll becomes unusable, in order to allow economic production of cubic polycrystalline silicon chunk by low-contamination tooling.
If steel rolls, made from low-alloy steel, for example, are used instead of rolls made from low-contamination material, the result is increased instances of metal contamination. This makes subsequent cleaning unavoidable.
The cleaning method of US 2010/0001106 A1 provides a preliminary cleaning procedure in at least one stage with an oxidizing cleaning solution comprising the compounds hydrofluoric acid, hydrochloric acid, and hydrogen peroxide, a main cleaning procedure in a further stage with a cleaning solution which comprises nitric acid and hydrofluoric acid, and a hydrophilization of the chunk polysilicon by means of a further stage with an oxidizing cleaning solution.
U.S. Pat. No. 7,270,706 B2 as well proposes corresponding, optional cleaning methods.
Such cleaning methods allow the instances of metal contamination on the surface to be reduced.
However, not all metals are readily soluble in the mineral acids used. It has emerged that tungsten—when tungsten carbide is used as roll material, the material becomes contaminated with tungsten—cannot be removed satisfactorily, because tungsten is not readily soluble in the mineral acids.
It has emerged, moreover, that the concentrations of dopant in the polysilicon chunks produced by the two methods described above are high: specifically, at least 100 ppta (dopants boron, phosphorus, and arsenic in total).
Arising from this problem scenario is a first objective of the invention. A further goal is to lower the acquisition costs for the cleaning unit by 25%, and the acid and disposal costs likewise by 25%, relative to the existing methods.